The present invention relates to an image sensing method, an image sensing apparatus, a lens control method therefor, and a storage medium storing a control program for controlling the image sensing apparatus.
FIG. 5 shows the schematic arrangement of an inner focus type lens system in a conventional image sensing apparatus. Referring to FIG. 5, reference numeral 501 denotes a first stationary lens; 502, a zoom lens for zooming; 503, a stop for light amount adjustment; 504, a second stationary lens; 505, a focus lens (focus compensation lens) having both a focus adjustment function and a so-called compensation function of correcting the movement of a focal plane upon zooming; and 506, the imaging plane of an image sensing element such as a CCD.
In the lens system having the arrangement shown in FIG. 5, since the focus compensation lens 505 has both the compensation function and the focus adjustment function, the position of the focus compensation lens 505 to be focused on the imaging plane 506 varies depending on the object distance even if the focal length remains the same.
FIG. 6 shows the result obtained by changing the object distance at each focal length, and continuously plotting the position of the focus compensation lens 505 to be focused on the imaging plane 506. Referring to FIG. 6, the ordinate indicates the position of the focus compensation lens 505; and abscissa, the focal length (the position of the zoom lens).
If one of the loci in FIG. 6 is selected in accordance with the object distance, and the focus compensation lens 505 is moved along the selected locus during zooming, zooming can be done without any blur.
A front-element focus type lens system has a compensation lens independently of a zoom lens. The zoom lens and the compensation lens are coupled to each other through a mechanical cam ring. Assume that this cam ring has a knob for manual zooming, and the focal length is to be changed manually. In this case, the cam ring rotates following the movement of the knob, and the zoom lens and the compensation lens move along the cam groove of the cam ring. As long as the focus lens stays focused, no blur is caused by the above operation.
In general, in control operation in the inner focus type lens system having the above characteristics, a plurality of pieces of locus information shown in FIG. 6 are stored in advance, in some form, in the lens control microcomputer, and a locus (cam locus) is selected on the basis of the positions of the focus lens and the zoom lens, thereby zooming along the selected locus.
To read out the position of the focus lens relative to the position of the zoom lens from the storage element and use it in lens control, the position of each lens must be read accurately to some degree. As is obvious from FIG. 6 as well, when the zoom lens moves at a constant speed or nearly constant speed, in particular, the slope of the locus of the focus lens changes from moment to moment with changes in focal length. This indicates that the moving speed and the slope of movement of the focus lens change from moment to moment. In other words, an actuator for the focus lens needs to have speed response characteristics with a high precision of 1 Hz to several hundred Hz.
As an actuator, i.e., a focus lens driving motor, in an inner focus type lens system, which satisfies the above requirement, a stepping motor is generally used. The stepping motor rotates in perfect synchronization with step pulses output from the lens control microcomputer or the like. Since the step angle of the motor per pulse is constant, high speed response characteristics, high stop precision, and high position precision can be obtained.
When a stepping motor is to be used, since the rotational angle is constant with respect to the number of step pulses, these pulses can be used as an incremental encoder. Hence, no special position encoder is required.
As described above, when zooming is to be performed by using a stepping motor while the object is kept in focus, the locus information in FIG. 6 must be stored in advance, in some form (loci themselves or a function using the lens position as a variable), in the lens control microcomputer or the like, and the focus lens must be moved on the basis of the locus information read out in accordance with the position or moving speed of the zoom lens.
FIGS. 7 and 8 are views for explaining an example of conventional locus tracking methods.
FIG. 7 shows the in-focus locus of the focus lens, accompanying the movement of the zoom lens, for each object distance. Each locus is specified by the zoom lens position, i.e., the focal length, and the focus lens position, i.e., the object distance. The object can be maintained in focus during zooming by driving the focus lens along the locus.
FIG. 8 shows how the cam locus information in FIG. 7 is stored in the lens control microcomputer (or external memory). Referring to FIG. 8, a variable v indicates the focus lens position (zone) in the object distance direction, and a variable n indicates the zoom lens position (zone) in the focal length direction. Data A(n, v) representing the focus lens position is specified by these pieces of information.
Referring to FIG. 7, the ordinates indicates the focus lens position; and the abscissa, the zoom lens position (z0, z1, z2, . . . , z6). In addition, L1(a0, a1, a2, . . . , a6) and L3(b0, b1, b2, . . . b6) are respectively the representative loci stored in the lens control microcomputer. L2(p0, p1, p2, . . . , p6) is the locus calculated on the basis of the above two loci L1 and L3 byp(n+1)=|p(n)−a(n)|/|b(n)−a(n)|x|b(n+1)−a(n+1)|+a(n+1)  (1)
According to equation (1), in the case shown in FIGS. 7 and 8, when the focus lens is located at p0, the ratio at which p0 internally divides a line segment b0–a0 is obtained, and a point that internally divides a line segment b1–a1 at the obtained ratio is defined as p1. The moving speed of the focus lens at which an in-focus state is maintained can be obtained from the difference in position between the points p1 and p0 and the time required for the zoom lens to move from z0 to z1.
Consider a case in which the zoom lens need not stop on only boundaries having the stored representative locus data.
FIG. 9 is a graph for explaining interpolation in the zoom lens position direction. This graph shows a portion of FIG. 7 with arbitrary zoom lens positions.
Referring to FIG. 9, the ordinate indicates the focus lens position, and the abscissa, the zoom lens position (Zk−1, Zx, Zk). In addition, L1(bk−1, bx, bk), L2(pk−1, px, pk), and L3(ak−1, ax, ak) indicate the representative loci (focus lens positions relative to zoom lens positions) stored in the lens control microcomputer. When the zoom lens positions are Z0, z1, . . . , Zk−1, zk, . . . , Zn, the corresponding focus lens positions for the respective object distances are:                a0, a1, . . . . , ak−1, ak, . . . , an        b0, b1, . . . , bk−1, bk, . . . , bn        
When the current zoom lens position is the position Zx, which is not on a zoom boundary, and the current focus lens position is the position px, ax and bx are given by:ax=ak−(Zk−Zx)×(ak−ak−1)/(Zk−Zk−1)  (2)bx=bk−(Zk−Zx)×(bk−bk−1)/(Zk−Zk−1)  (3)That is, ax and bx can be obtained by internally dividing data of the four stored representative locus data (ak, ak−1, bk, and bk−1 in FIG. 9), which are obtained at the same object distance, at the internal ratio obtained from the current zoom lens position and two zoom boundary positions (e.g., Zk and Zk−1 in FIG. 9) on the two sides of the current zoom lens position. Then, pk and pk−1 can be obtained by internally dividing data of the four stored representative data (ak, ak−1, bk, and bk−1 in FIG. 9), which are obtained at the same focal length, at the internal ratio obtained from ax, px, and bx, as per equation (1). When zooming is to be performed from the wide-angle side to the telephoto side, the moving speed of the focus lens at which an in-focus state is maintained can be obtained from the difference in position between the destination focus position pk and the current focus position px and the time required for the zoom lens to move from Zx to Zk. When zooming is to be performed from the telephoto side to the wide-angle side, the moving speed of the focus lens at which an in-focus state is maintained can be obtained from the difference in position between the target focus position pk−1 and the current focus position px and the time required for the zoom lens to move from Zx to Zk−1.
The above locus tracking method has been proposed.
Again, as is apparent from FIG. 6, in particular, when the zoom lens moves from the telephoto side to the wide-angle side, the object can be maintained in focus by the above locus tracking method. When, however, the zoom lens moves from the wide-angle side to the telephoto side, since a specific locus which the focus lens at a convergence point should track is not known, an in-focus state cannot be maintained by the same locus tracking method.
FIGS. 10A and 10B are graphs for explaining an example of locus tracking which has been proposed to solve the above problem. Referring to FIG. 10A, the ordinate indicates the high-frequency component (sharpness signal) of a luminance signal, i.e., an AF evaluation signal, in the vertical sync period, and the abscissa, the position of the zoom lens. Referring to FIG. 10B, the ordinate indicates the position of the focus lens; and the abscissa, the position of the zoom lens.
Referring to FIG. 10A, reference numeral 1001 denotes the maximum value of the level of the sharpness signal; 1002, the minimum value (TH1) of the level of the sharpness signal; and 1003, the level of the sharpness.
Referring to FIG. 10B, reference numerals 1004 and 1005 denote cam loci; and 1006, the zoom lens position.
Assume that the cam locus 1004 in FIG. 10B is a focus cam locus in zooming for a given object. Assume that the focus cam locus tracking speeds on the wide-angle side relative to the zoom lens position 1006 (Z14) are positive (in the direction of the closest focusing distance of the focus lens), and the in-focus cam tracking speeds on the telephoto side relative to the zoom lens position 1006 (Z14) on which the lens moves toward infinity are negative. Admittedly, when the focus lens moves along the focus cam locus 1004 while it stays focused, the level of the above sharpness signal becomes almost constant.
The moving speed of the focus lens that traces the focus cam locus 1004 in FIG. 10B in zooming is represented by Vf0, and the actual moving speed of the focus lens is represented by Vf. When zooming is performed while the moving speed Vf of the focus lens is changed with respect to the moving speed Vf0 of the focus lens that traces the focus cam locus 1004, the resultant cam locus becomes the zigzag cam locus 1005. At this time, the level of the sharpness signal changes to produce valleys and crests, as indicated by “1003” in FIG. 10A. At each point of intersection of the cam loci 1004 and 1005, the level 1003 of the sharpness signal becomes maximum (even points of Z0, z1, . . . , Z16). At each odd point of Z0, Z1, . . . , Z16, at which the moving direction vector of the cam locus 1005 is switched, the level 1003 of the sharpness signal becomes minimum.
Referring to FIG. 10A, although reference numeral 1002 denotes the minimum value (TH1) of the level 1003 of the sharpness signal, a minimum value (TH1) 1002 may be set for the level 1003 of the sharpness signal, and the moving direction vector of the cam locus 1005 in FIG. 10B may be switched every time the level 1003 of the sharpness signal becomes the minimum value (TH1) 1002. With this operation, the moving direction of the focus lens upon switching can be set in a direction in which the lens approaches the focus cam locus 1004 in FIG. 10B.
That is, zooming with less blur amount can be performed by controlling the moving direction and speed of the focus lens to reduce blur every time an image blurs by the difference between the maximum level 1001 and minimum level (TH1) 1002 of the sharpness signal.
With the use of the above method, in zooming from the wide-angle side to the telephoto side, in which the cam loci in FIG. 6 diverge from a convergence point, a locus along which the level 1003 of the sharpness signal does not become lower than the minimum value (TH1) 1002 in FIG. 10A, i.e., blur does not occur beyond a predetermined amount, can be selected by repeating switching operation (in accordance with changes in the level of the sharpness signal), like the cam locus 1005 in FIG. 10B, while controlling the moving speed Vf of the focus lens with respect to the tracking speed described in the prior art (calculated by using p(n+1) obtained from equation (1)) even if the focusing speed Vf0 is unknown.
In this case, the moving speed Vf of the focus lens is given by:Vf=Vf0+Vf+  (4)Vf=Vf0+Vf−  (5)where Vf+ is the correction speed in the positive direction, and Vf− is the correction speed in the negative direction.
In this case, to prevent offsets in locus tracking operation by the above zooming method, the correction speeds Vf+ and Vf− are determined such that the internal angle the two direction vectors of the moving speed Vf of the focus lens, which are obtained from equations (4) and (5), make with each other is equally divided into two by the direction vector of the moving speed Vf0 of the focus lens that traces the focus cam locus 1004. In addition, a technique of improving locus selection precision by changing the magnitudes of the correction speeds in accordance with focal length and depth of field has also been proposed. In addition, a method of tracking a locus by using an integral signal that sensitively changes with blur in place of a sharpness signal has been proposed.
In the prior art described above, the sharpness signal is the high-frequency component of a video signal. At a shutter speed of 1/60 sec in the general NTSC scheme, a video signal can be obtained in each vertical sync period, as shown in FIG. 11A. When, however, the shutter speed decreases, and the charge storage time of the image sensing element 506 is prolonged, for example, when a shutter speed of 1/30 sec is set as shown in FIG. 11B, a signal stored in the image sensing element 506 for 1/30 sec becomes a video signal. Therefore, a video signal is detected once per every 1/30 sec, i.e., once per every two vertical sync periods. That is, no video signal is detected once per every two vertical sync periods. At a shutter speed of 1/15 sec, a video signal is detected only once per every four vertical sync periods.
FIGS. 12A to 12D show the relationship between the sharpness signal and the elapsed time at a shutter speed of 1/15 sec in an in-focus state in which the positions of an object and the focus lens remain the same. FIG. 12A shows a vertical sync signal; FIG. 12B, timing information from shutter speed control; FIG. 12C, a video signal; and FIG. 12D, a sharpness signal.
As can be seen from FIGS. 12A to 12D, the level of the sharpness signal changes even as the lens stays focused. According to the method of controlling the moving direction and speed of the focus lens to reduce blur every time an image blurs by the difference between the maximum value 1001 and minimum value (TH1) 1002 of the level 1003 of the sharpness signal in FIGS. 10A and 10B as in zooming operation, zooming operation following the object is realized in accordance with the difference information between the maximum value 1001 and minimum value (TH1) 1002 of the sharpness signal level 1003. When the shutter speed is low, e.g., 1/15 sec, information of the minimum value (TH1) 1002, from which a sharpness signal is obtained, is obtained once per every four vertical sync periods. That is, the period at which the direction is changed following the object is prolonged. As a result, the object cannot be tracked during zooming.
In addition, multiple speed zooming has recently become popular, and hence the zoom speeds vary from low-speed zooming to high-speed zooming. In this case as well, the focal plane must track an object to be photographed during zooming.
Since the sharpness integral signal of an AF evaluation signal is a value obtained by adding sharpness peak signals in a horizontal sync period together within a vertical sync period, changes in evaluation signal are also integrated and appear owing to noise caused by camera shakes or the like during low-speed zooming. For this reason, whether a given change is a change in evaluation signal due to a focus lens tracking error during zooming or a change due to noise or the like cannot be determined. As a result, the focal plane cannot accurately track the object during zooming.
Furthermore, in high-speed zooming, since the view angle changes steeply, whether a given change is a change in evaluation signal due to a focus lens tracking error during zooming or a change in view angle cannot be determined. As a result, the focal plane cannot accurately track the object during zooming.